1. Field of the Invention
This invention relates to bumped-beam tape for automatic gang bonding to semiconductors. More specifically it refers to a process for producing bumped-beam tape by mechanically forming bumps on a thin copper substrate.
2. Description of the Prior Art
As recognized in an article appearing in Insulation/Circuits, September 1979, pages 65-66, "The problem of putting the bumps on the tape is formidable. The bump must be approximately one mil high, two mils in diameter, and well enough indexed on the end of the fragile cantilevered beam that it will enter the well over the pad of a semiconductor chip when depressed by the bonding tool" . . . "Finally, the bump location must be carefully controlled dimensionally, and the bump must be formed at a reasonable cost."
The article goes on to point out that in two layer tape using polyimide as one layer, one means of copper buildup is by selectively plating the surface of the beam. The difficulty with this procedure is the plating of a two mil bump, one mil high in a two mil hole in a resist. Although bumps were formed this way they proved to be difficult to control in dimension and did not have a good adhesion to the original copper base due to difficulty of removing contamination remaining from the polyimide layer. The other method to form bumps is to plate up a thick copper layer and then etch it away leaving merely a thin beam and a bump at the end. The article states on page 67, "This can lead to bumps of very interesting shapes, since the different copper layers may etch at different rates." Such a difference in shapes is undesirable from a quality control viewpoint.
In U.S. Pat. No. 4,209,355 the process of etching away a beam to form a finger is described. U.S. Pat. No. 3,838,984 describes a method of laminating polyimide films to copper foil and depositing gold bumps on the end of fingers on the foil. U.S. Pat. No. 4,063,993 describes a method of forming bumps by using a gang bonding tool made of carbon, heated to a temperature of 550.degree. C. to press the inner ends of the interconnect beads down against a gang bonding bump with a pressure of approximately 8 grams per square mil for a time of approximately 0.2 seconds. This is an additional step in the process that is costly. U.S. Pat. No. 4,125,441 describes a method of electro forming a bump circuit. All these prior art methods suffer from a disadvantage of not being able to prepare a bump geometrically having a controlled height, diameter and radius. This is critical since the optimum bump should be dome shaped on its exterior. A flat end on the bump results in a bigger footprint and a decreased unit loading when it is attached to the semiconductor device.